As a conventional surface acoustic filter device, particularly as a duplexor filter of an antenna for mobile communication equipment, a SAW filter of a three electrode structure is typically employed, which has a central electrode used as an output electrode and electrodes connected electrically in parallel and used as input electrodes on both sides of the output electrode. The reason why this three electrode structure is adopted is for reducing an amount of reflection at the electrodes due to both radiation and an amount of loss due to the two-way characteristic of the SAW transmitter-receiver electrodes by means of matching of the central electrode, and when the two electrodes on the both outer sides operating as an input electrode and the central electrode operating as an output electrode are made to match external circuits in the input and output sides respectively, the loss due to above-mentioned two-way characteristics can be reduced to 3 dB as described in, for example, Electronics Letters, Vol. 8 (23), p. 553 (1970). On the other hand, in order to reduce out-band responses of a filter, electrodes on the outer sides are weighted in amplitude and the like as seen in, for example, Technical Report of the Institute of Electronics and Communication Engineers, Japan, MW 82-4. In order to further reduce the above-mentioned two-way loss, the above-mentioned three electrode structure has been developed, and a SAW filter in which a plurality of transmitting electrodes and receiving electrodes are alternately disposed on the same propagation path and disposed in symmetry with each other is employed as described in, for example, U.S. Pat. No. 3,582,840.
In the case where the SAW filter having the above-mentioned conventional structure was used as a filter on the receiving side (the first stage) of a duplexer filter, there was a problem that since a high power transmitting signal (frequency f.sub.T) was incident on the out-band of this filter, when a weak out-band spurious signal (frequency f.sub.SP) entered via an antenna into this filter, the spurious signal and the transmitting signal were coupled to produce intermodulation signals due to the non-linear characteristics of the filter, one of which formed a signal in a receiving band (f.sub.R), providing disturbances to speech.
The occurrence of the above-mentioned problem is suggested as results of experiments on inter-modulation characteristics using the SAW filter based on the above-mentioned first literature on the Pre-Print of the Institute of Electronics and Communication Engineers, Japan, 1982 Conference 8-161 (1983).